This invention relates to a motion adaptive type chrominance signal mixing circuit for detecting a luminance signal by mixing color signals which are obtained through a band pass filter, 2H comb filter or line comb filter, and frame comb filter in accordance with a motion amount of a video signal, and subtracting the mixed color signals from an original composite video signal, so as to separate the luminance signal from the composite video signal in a color video signal processing system. And more particularly, the present invention relates to a color signal mixing circuit and a method thereof for improving the picture quality effected by removing an element resulting in image quality degradation included in a color signal component processed by a frame comb filter when the mixing of the motion adaptive type color signal is performed with a semi-motion picture in a motion adaptive type color signal mixing circuit.
In a conventional luminance/chrominance signal separator within a video signal processor, a motion adaptive type chroma mixing circuit for detecting a luminance signal detects and counts the motion data of a picture, and classifies them into a motion picture, a semi-motion picture and a still picture. Then, the chroma mixing circuit outputs a chroma signal processed by a line comb filter in case of the motion picture, while it outputs a chroma signal processed by a frame comb filter in case of the still picture. Also, the chroma mixing circuit outputs a chroma signal which is obtained by mixing properly chroma signals processed by a line comb filter and a frame comb filter in accordance with a motion factor of a video signal.
Assuming that a coefficient of the motion factor with n bits is a value of K, a line comb filtered chroma signal is a value of C.sub.L, a frame comb filtered chroma signal is a value of C.sub.F, and a motion adaptive type mixed chroma signal is a value of C.sub.M, a motion adaptive type mixing operation is represented by the following equation: ##EQU1## Therefore, a motion adaptive type chroma mixing circuit is made up of two adder 101 and 103, and a multiplier 102 for operation of the coefficient K and the respective line and frame combed chroma signals C.sub.L and C.sub.F as shown in FIG. 3. Referring to FIG. 3, when a line combed chroma signal C.sub.L is inputted to the first adder 101, and a frame combed chroma signal C.sub.F is inverted and inputted, an output of the first adder 101 is inputted to the multiplier 102, multiplied by the coefficient K, and supplied to the second adder 103. Then, the second adder 103 adds the the out of multiplier 102 to the frame combed chroma signal C.sub.F and outputs a motion adaptive type chroma mixed signal.
Here, there may, in another prior art, be a case that a band pass filter which is not shown in FIG. 3 is connected to one terminal of the first adder 101, and therefore a frame combed chroma signal C.sub.F is filtered by the band pass filter before inputted to the first adder 101.
However, in the former motion adaptive type chroma mixing circuit as described above, an image quality is not degraded in case of a complete still picture and a complete motion picture, but an image quality is degraded due to the fact that the frame combed chroma signal includes many error components resulting from crosstalk when the frame combed chroma signal C.sub.F and the line combed chroma signal C.sub.L are mixed in case of semi-motion picture.
In addition, in the latter case with a band pass filter which is connected to one terminal of the first adder for filtering the frame combed chroma signal C.sub.F, the frame combed chroma signal C.sub.F is again filtered by a band pass filter in a complete still picture.
This may result in image quality degradation. Therefore, there has been required to selectively control a band pass filter to be inserted in or removed from one terminal of the first adder in accordance with the motion data of a picture.